Benign prostatic hyperplasia (BPH) is the most common neoplasm diagnosed in men of the United States and causes considerable morbidity and mortality. Prostate size varies considerably among normal aging men. Enlargement of the transitional zone that is primarily responsible for urethral obstruction also varies considerably. Although its cause is unknown, there is evidence that endocrine, genetic and environmental factors are all important in its pathogenesis. We have established that hereditary and environmental factors influence the variation in sex hormone levels in male twins. We hypothesize that prostate size, shape, and nodularity and residual urine volume are affected by endocrine, genetic and environmental factors. We propose to conduct studies of these factors in 120 monozygotic and 120 dizygotic male twin pairs between the ages of 25 to 75 years. A questionnaire will obtain data on demographic factors, medical history, sexual development, diet, and symptoms of urinary obstruction. Rectal examination, transrectal ultrasound (which will measure size of the transitional zone and total prostate size and residual urine volume), and anthropometry will be done by the team. Plasma sex steroids, insulin, and prostate specific antigen will be measured by immunoassays. Hereditary, endocrine, and environmental influences on prostate size and shape and residual urine volume will be assessed with the twin model. Multiple linear regression will be used to ascertain the relationships among the variables measured and prostate morphometrics, symptoms, and residual urine volume. This study will be the first to examine both hereditary and environmental factors in the pathogenesis of BPH. It is made possible because of cooperative population for the study and the combined efforts of the investigative team.